The present invention is directed to a hybrid vehicle with an internal combustion engine that is assisted by an electric motor, and which uses the electric motor to assist in deceleration of the vehicle by way of regenerative braking. In particular, the invention provides a method and apparatus for controlling the state of charge of the traction battery which is utilized in hybrid vehicles to supply operating power to the electric motor.
In hybrid vehicles of the type referred to, it is extremely important to maintain the state of charge of the traction battery within certain limits in order to optimize both acceleration assist, regenerative deceleration and traction battery life. That is, if the state of charge of the battery is too high, it will be unable to absorb the energy generated by regenerative braking of the vehicle. On the other hand, if the state of charge is too low, insufficient power will be available to provide maximum assistance to the internal combustion engine in an acceleration of the vehicle. Accordingly, it is necessary that the state of charge be regulated in a manner which maintains it within an optimum range.
In vehicles of the type described, the typical method for state of charge maintenance is to control the amount of acceleration assist and regenerative braking during driving of the vehicle. That is, the amount of regenerative braking and acceleration assist which are performed is controlled in order to control the voltage or state of charge of the battery to a setpoint or range, which is determined primarily as a function of battery and system requirements. During those time periods when neither regenerative braking nor acceleration assist is being performed, the electric motor is used in a generator mode to charge the battery.
One difficulty with the typical method for state of charge maintenance is that static loads, such as a DC to DC converter, onboard AC power generation, electric power steering or other traction battery loads discharge the battery pack even when the vehicle is not being driven. Accordingly, it is possible for the battery to be discharged to such a degree that it is unable to provide optimum acceleration assist, as required for operation of the hybrid vehicle.
One object of the present invention is to provide an intelligent battery regulation arrangement which directly compensates for the effect of static loads on the traction battery.
Another object of the invention is to provide a battery regulation algorithm which prevents the state of charge of the battery from falling below optimum limits when the vehicle is not being driven.
Still another object of the invention is to provide a battery state of charge control method which intelligently transitions between voltage regulation control and hybrid traction torque control.
These and other objects and advantages are achieved by the intelligent battery voltage regulation algorithm according to the invention, in which electric motor is operated in a generating mode to charge the vehicle battery during time periods when neither hybrid assist nor regenerative braking is active. According to the invention, during such time periods closed loop control is performed such that the electric motor torque (that is, the torque applied by the electric motor operating in the generating mode) provides a traction battery charge current which is then used to regulate the traction battery voltage to a specific setpoint. This occurs frequently when the vehicle is at rest (and hence, neither regenerative braking nor acceleration assist is necessary), but may also take place when the vehicle is being driven at a constant speed and load.
The voltage regulation setpoint is selected such that sufficient battery power is available for hybrid assist, and at the same time, sufficient charge capacity is available to absorb energy generated by regenerative braking. The regulation setpoint is also based on battery temperatures, and whether AC power generation is active. Thus, the control algorithm according to the invention directly compensates for static loads by controlling the electric motor to operate in a generator mode (for example, using P, P.I. or P.I.D. feedback control) during those time periods when neither regenerative braking nor hybrid assist is taking place. (Other forms of control, such as feed forward or open loop control are of course also possible.)
It should be noted that the maximum electric motor torque request by the intelligent battery voltage regulation algorithm according to the invention is limited so that the engine idle quality is not adversely effected. The torque request may also decrease with increasing engine RPM.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.